1. Field of the Invention
The present invention relates to a radio-frequency tuning circuit adapted to be used for RF stage of a superheterodyne receiver, thereby suppressing generation of second harmonic signal.
2. Description of the Prior Art
In order to have a better understanding of the present invention, description will first be made with reference to FIGS. 5 and 6 of the accompanying drawings, which illustrate the circuit arrangement of conventional radio-frequency tuning circuit and frequency versus attenuation characteristics thereof respectively.
As will be seen, in the conventional radio-frequency tuning circuit shown in FIG. 5, an input tuning transformer T7 and output tuning transformer T8 are connected together in such a manner as to constitute a double-tuning circuit.
The input tuning transformer T7 comprises a primary coil L18, and a secondary coil L19. A capacitor C7, and a pair of variable-capacitance diodes comprising variable-capacitance diodes D10 and D11 having their cathodes connected together, are connected in parallel with the secondary coil L19 of the input tuning transformer T7. Thus, a tuning circuit 51 is formed by the secondary coil L19, variable-capacitance diodes D10, D11, and capacitor C7.
Further, the output transformer T8 comprises a primary coil L20, and secondary coil L21. A variable-capacitance diode D12 is connected to the primary coil L20 of the output tuning transformer T8, and a capacitor C8 is connected in parallel with the primary coil L20 of the output tuning transformer T8. Thus, a further tuning circuit 52 is formed by the primary coil L20, variable-capacitance diode D12, and capacitor C8.
The input tuning transformer T7 has the secondary coil L19 thereof tied to a tap of the primary coil L20 of the output tuning transformer T8 so that the tuning circuits 51 and 52 constitute the double-tuning circuit. R7 and R8 are bias resistors.
With the above conventional radio-frequency tuning circuit, signal derived from antenna circuit (not shown) is inputted to the input tuning transformer T7, and thus the signal as inputted is transferred from the primary coil L18 to the secondary coil L19 so that a signal of receiving frequency is selected at the tuning circuit 51. The signal as selected at the tuning circuit 51 is then transferred to the primary coil L20 of the output tuning transformer T8 so that signal of the receiving frequency is selected at the tuning circuit 52, the signal which in turn is transferred to and outputted from the secondary coil L21 of the output tuning transformer T8.
The input tuning transformer T7 and output tuning transformer T8 which constitute the conventional radio-frequency tuning circuit are ones exhibiting identical Q-factor. Further, the variable-capacitance diodes D10, D11, and D12 are ones having identical voltage versus capacitance characteristics. Still further, only the tuning circuit 51 uses a pair of variable-capacitance diodes, or the variable-capacitance diodes D10 and D11 having the cathodes thereof connected together, for the purpose of suppressing generation of the second harmonic signal which tends to be caused because of the voltage versus capacitance characteristics of the variable-capacitance diodes being non-linear.
The resonant impedance of the tuning circuit 51 is given by EQU Z.sub.7 =.omega.QT.sub.7 /TC.sub.7 ( 1)
where QT.sub.7 is the Q-factor of the input tuning transformer T.sub.7 ; TC.sub.7 is the combined capacitance of the tuning circuit 51; and .omega. represents 2.pi.f.
The resonant impedance of the tuning circuit 52 is given by EQU Z.sub.8 =.omega.QT.sub.8 /TC.sub.8 ( 2)
where QT.sub.8 is the Q-factor of the output tuning transformer T.sub.8 ; and TC.sub.8 is the combined capacitance of the tuning circuit 52; and .omega. represents 2.pi.f.
As mentioned above, the variable-capacitance diodes D.sub.10, D.sub.11 and D.sub.12 have identical voltage versus capacitance characteristics; and only the tuning circuit 51 uses a pair of variable-capacitance diodes (more specifically, two variable-capacitance diodes connected in series with each other); thus, the combined capacitance of the tuning circuit 52 is twice as high as that of the tuning circuit 51, and equation (2) can be rewritten as follows: EQU Z.sub.8 =.omega.QT.sub.8 /2TC.sub.7 ( 3).
Further, also as mentioned above, the input tuning transformer T.sub.7 and output tuning transformer T.sub.8 exhibit identical Q-factor; thus, the resonant impedance of the tuning circuit 52 and that of the tuning circuit 51 can be expressed as follows: EQU Z.sub.7 /Z.sub.8 =2 (4)
As will be seen, with the above conventional radio-frequency tuning circuit, despite the fact that the Q-factor of the input transformer T.sub.7 and that of the output tuning transformer T.sub.8 are equal to each other, the combined capacitance of the tuning circuit 52 is twice as high as that of the tuning circuit 51 so that the resonant impedance of tuning circuit 52 turns out to be twice as high as that of the tuning circuit 51. Thus, such difference in resonant impedance between the tuning circuits 51 and 52 results in the Q-factors of these tuning circuits being different from each other.
Thus, the radio-frequency tuning circuit including the double-tuning circuit constituted by the tuning circuits 51 and 52 turns out to be a circuit which is poor in terms of Q-factor balance. Another disadvantage of the above conventional radio-frequency tuning circuit is such that because of the fact that the bandwidth thereof, is broadened as shown at 63 in FIG. 6 for the purpose of eliminating sensitivity difference which tends to be caused due to tracking error, the circuit is susceptible to occurrence of ripple as shown by the solid characteristic curve 61 in FIG. 6.
When ripple occurs in the characteristics of the radio-frequency tuning circuit as mentioned above, frequency F1 at which attenuation is minimized is lower than the center frequency F0 of such characteristics with no ripple as shown by the dotted curve 62. This means that the above conventional radio-frequency tuning circuit has such a drawback that when the frequency F1 for minimum attenuation is deviated from the receiving frequency due to variation in temperature and/or humidity, the signal of the receiving frequency tends to be attenuated so that the sensitivity is deteriorated.
Furthermore, with the radio-frequency tuning circuit having such characteristics, when it is tuned to receive a signal of 1010 KHz for example, there is a tendency that the frequency F1 for minimum attenuation is brought into registration with 1010 KHz; thus the bandwidth 63 becomes broader at a portion higher than the frequency F1, so that assuming that signals of 1030 KHz and 1050 KHz, both of which fall within the bandwidth, are inputted, these signals are transferred without being attenuated. That is, despite the attempt to suppress second harmonic interference at the tuning circuit 51, the signal of 1030 KHz is permitted to reach the tuning circuit 52, so that the second harmonic of the 1030 KHz signal, i.e., a signal of 2060 KHz is generated at the tuning circuit 52. This gives rise to a problem that there is caused such intermodulation that the second harmonic signal of 2060 KHz and the signal of 1050 KHz are combined, so that a signal of 1010 KHz is generated.